Antenna allocation apparatus and method for cellular mobile communication system

ABSTRACT

A method and apparatus for selecting and allocating antennas efficiently are provided. The method includes transmitting, to a User Equipment (UE), information indicating a configuration of a plurality of Channel Status Information Reference Signals (CSI-RSs) through UE specific signaling during an initial access attempt with the UE; receiving, from the UE, CSI-RS measurement results indicating configured CSI-RSs; transmitting CSI-RSs corresponding to a set of available distributed ports (D-ports) based on received signal strength information included in results of the CSI-RS measurement; and determining a CSI-RS of a selected D-port set for use in communications based on feedback information received from the UE.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) to KoreanApplication Serial No. 10-2010-0116020, which was filed in the KoreanIntellectual Property Office on Nov. 22, 2010, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to allocating antennas in acommunication system, and more specifically, to a method and apparatusfor selecting and allocating antennas efficiently in a cellular mobilecommunication system based on a Distributed Antenna System (DAS).

2. Description of the Related Art

Mobile communication systems have evolved into a high-speed,high-quality wireless packet data communication systems that providedata and multimedia services in addition to the voice-oriented servicesprovided through early mobile communication systems. Recently, variousmobile communication standards, such as High Speed Downlink PacketAccess (HSDPA), High Speed Uplink Packet Access (HSUPA), Long TermEvolution (LTE), and LTE-Advanced (LTE-A) defined in 3^(rd) GenerationPartnership Project (3GPP), High Rate Packet Data (HRPD) defined in3^(rd) Generation Partnership Project-2 (3GPP2), and 802.16 defined inIEEE, have been developed to support such high-speed, high-qualitywireless packet data communication services. In particular, LTE has beenis a technology capable of facilitating such high speed packet datatransmission and maximizing the throughput of the radio communicationsystem with various radio access technologies. LTE-Advanced (LTE-A) isan evolved version of LTE that improves the data transmissioncapabilities of LTE.

Existing 3^(rd) generation wireless packet data communication systems,such as High-Speed Downlink Packet Access (HSDPA), High-Speed UplinkPacket Access (HSUPA) and High-Rate Packet Data (HRPD) systems, usetechnologies such as Adaptive Modulation and Coding (AMC) andChannel-Sensitive Scheduling to improve transmission efficiency. Throughthe use of AMC, a transmitter can adjust an amount of transmission dataaccording to a channel state. When the channel state is below a certainquality level (i.e., a ‘Poor’ channel state), the transmitter reducesthe amount of transmission data to adjust the reception errorprobability to a desired level, and when the channel state is at orabove a certain quality level (i.e., a “Good” channel state), thetransmitter increases the amount of transmission data to adjust thereception error probability to the desired level, thereby efficientlytransmitting a large volume of information. With the use of theChannel-Sensitive Scheduling-based resource management method, thetransmitter selectively provides services to a user having a betterchannel state from amongst several users, thus increasing the systemcapacity, in contrast to methods that include allocating a channel toone user and servicing the user with the allocated channel. Thiscapacity increase is referred to as multi-user diversity gain. The AMCtechnique and the Channel-Sensitive Scheduling methods each includeapplying an appropriate modulation and coding scheme at a most-efficienttime determined according to partial channel state information fed backfrom a receiver.

In conjunction with a Multiple Input and Multiple Output (MIMO) scheme,the AMC technique can be used to determine a number of spatial layersfor transmission or rank. When using the AMC technique in this manner,the AMC scheme is implemented in consideration of the number of layersto be used in MIMO transmission as well as a coding rate and modulationlevel.

Meanwhile, research is being conducted in order to find ways to replacethe Code Division Multiple Access (CDMA) as the multiple access schemeof the 2^(nd) and 3^(rd) generation mobile communication systems forOrthogonal Frequency Division Multiple Access (OFDMA) in next generationsystems. 3GPP and 3GPP2 have started standardization of evolved systemsusing OFDMA. OFDMA utilizes a larger system capacity than a systemcapacity utilized through CDMA. One of the significant factorscontributing the increase of system capacity of OFDMA relative to CDMAis the use of frequency domain scheduling. Similar to the channelsensitive scheduling based on the time-varying characteristic ofchannels, it is possible to obtain more capacity gain by using thefrequency-varying characteristic of the channels.

In conventional technologies, the cellular system is configured with aplurality of cells as shown in FIG. 1 in order to provide mobilecommunication with the aforementioned techniques.

FIG. 1 is a schematic diagram illustrating a cellular system includingthree cells each centered around an antenna.

Referring to FIG. 1, a cellular system includes three cells 100, 110,and 120, and reference numeral 160 denotes an exemplary configuration ofthe cell 100. The cell 100 is centered around the antenna 130 and servesUser Equipments (UEs) 140 and 150 in its coverage area. The antenna 130provides the UEs 140 and 150 located in the cell 100 with a mobilecommunication service. The UE 140 is located further away from theantenna 130 than the UE 150, such that the UE 140 is served by theantenna 130 at a lower data rate than the UE 130.

As shown in FIG. 1, each cell is configured in the form of a CentralAntenna (CAS) antenna system in which the cell is centered around theantenna. In CAS, although multiple antennas are allocated to each cell,the antennas are arranged at the center of the cell to serve the UEs inthe service area. In case that antennas in each cell of a cellularmobile communication system are arranged and managed in the form of CASas shown in FIG. 1, it is necessary to transmit reference signals formeasuring downlink channel condition for each cell. In a 3GPP LTE-Asystem, a UE measures the channel status between the UE and an evolvedNode B (eNB) using a Channel Status Information Reference Signal(CSI-RS) transmitted by the eNB.

FIG. 2 is a diagram illustrating a configuration of a resource blockincluding CSI-RSs transmitted by the eNB.

Referring to FIG. 2, reference numerals 200 to 219 denote pairedpositions paired for signals of two CSI-RS antenna ports. For example,the eNB transmits the downlink estimation signals for two CSI-RS antennaports at the position 200. When the cellular system includes of aplurality of cells, such as in the example shown in FIG. 2, the CSI-RScan be transmitted at the positions allocated for each cell. Forexample, the cellular system can be configured such that the cell 100 ofFIG. 1 transmits CSI-RS at positions 200 of FIG. 2, while the cell 110transmits CSI-RS at positions 205, and the cell 120 transmits CSI-RS atpositions 210.

The different time-frequency resources are allocated for CSI-RStransmission of different cells in order to prevent the CSI-RSs of thedifferent cells from interfering with each other.

When using the CAS method as shown in FIG. 1, the transmit/receiveantennas of each eNB are concentrated at the center of the cell suchthat, there are limited capabilities for serving UEs located at the celledge at a high date rate. Therefore, the data rate for providing thecommunication service to the UE within the CAS-based cell is determinedsignificantly according to the location of the UE. In this respect, inconventional cellular mobile communication systems operating with callscentered around antennas, UEs located at cell edges cannot beeffectively served. Meanwhile, in such conventional cellular mobilecommunication systems, UEs located near the center of cells cancommunicate at a high data rate.

SUMMARY OF THE INVENTION

In order to solve the above-described problems, embodiments of thepresent invention provides an antenna allocation method and apparatusthat is capable of improving system performance by configuringDistributed Antenna System (DAS) and selecting and allocating thedistributed antennas efficiently.

According to an aspect of the present invention, a method for allocatingantennas performed by a base station in a cellular mobile communicationsystem is provided. The method includes transmitting, to a UserEquipment (UE), information indicating a configuration of a plurality ofChannel Status Information Reference Signals (CSI-RSs) through UEspecific signaling during an initial access attempt with the UE;receiving, from the UE, CSI-RS measurement results indicating configuredCSI-RSs; transmitting CSI-RSs corresponding to a set of availabledistributed ports (D-ports) based on received signal strengthinformation included in results of the CSI-RS measurement; anddetermining a CSI-RS of a selected D-port set for use in communicationsbased on feedback information received from the UE.

According to another aspect of the present invention, a method forallocating antennas performed by a User Equipment (UE) in a cellularcommunication system is provided. The method includes receiving, from abase station through UE-specific signaling after an initial accessattempt with the base station, information indicating a configuration ofa plurality of Channel Status Information Reference Signals (CSI-RSs);transmitting, to the base station, received signal strengths of CSI-RSsthat are measured based on the received information indicating theconfiguration; receiving, from the base station CSI-RSs of a set ofavailable distributed ports (D-ports); feeding back, to the basestation, results of measurements of CSI-RSs of the D-port set; andreceiving, through data communication with the base station, downlinkdata transmitted through the D-port set.

According to another aspect of the present invention, a method forallocating antennas performed a base station in a cellular mobilecommunication system is provided. The method includes sending, to a UserEquipment (UE), a request for transmission of a Sounding ReferenceSignal (SRS) during an initial access attempt; measuring signal strengthof the requested SRS received through a Central port (C-port) andDistributed ports (D-ports); transmitting, through UE-specificsignaling, information indicating a configuration of Channel StatusInformation Reference Signals (CSI-RSs) of available an D-port setconfigured based on the measured signal strength and transmission powerof the SRS; and determining a CSI-RS of a selected D-port set based onfeedback information received from the UE.

According to another aspect of the present invention, a method forallocating antennas performed by a User Equipment (UE) in a cellularmobile communication system is provided. The method includestransmitting a Sounding Reference Signal (SRS) in response to an SRSrequest received from a base station after an initial access attempt;and transmitting, when Channel Status Information Reference Signals(CSI-RSs) of available distributed ports (D-port set) is receivedthrough UE-specific signaling, feedback information for selectingantennas.

According to still another aspect of the present invention, a cellularmobile communication system is provided. The system includes a UserEquipment (UE) for receiving information indicating a configuration of aplurality of CSI-RSs from a base station through UE-specific signalingafter initial access attempt, measuring received signal strengths of theconfigured CSI-RSs, transmitting measured signal strengths to the basestation, receiving an available Distributed port (D-port) set from thebase station, and measuring CSI-RSs of the available D-port set; and abase station for transmitting the information indicating theconfiguration of the plurality of CSI-RSs to the UE through UE-specificsignaling during the initial access attempt, analyzing the CSI-RSsreceived from the UE, configuring the CSI-RSs of the available D-portset based on a result of the analysis, transmitting the CSI-RSs of theavailable D-port to the UE, and determining CSI-RSs of a selected D-portset used for communication based on feedback information received fromthe UE.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram illustrating a cellular system includingthree cells each centered around an antenna;

FIG. 2 is a diagram illustrating a configuration of a resource blockincluding CSI-RSs transmitted by the eNB;

FIG. 3 is a diagram illustrating a configuration of a distributedantenna system-based cellular mobile communication system having thetransmit/receive antennas distributed on a per-cell-basis according toan embodiment of the present invention;

FIG. 4 is a diagram illustrating the configuration of a resource blockfor allocating CSI-RS resources to multiple antennas belonging to a cellin a DAS-based system according to an embodiment of the presentinvention;

FIG. 5 is a diagram illustrating signaling between an eNB and a UE forallocating D-ports in the DAS-based LTE-A system according to anembodiment of the present invention;

FIG. 6 is a flowchart illustrating a procedure performed by an eNB forallocating D-ports to a UE in a DAS-based LTE-A system according to anembodiment of the present invention;

FIG. 7 is a flowchart illustrating a procedure performed by an eNB forallocating D-ports to a UE in a DAS-based LTE-A system according toanother embodiment of the present invention; and

FIG. 8 is a flowchart illustrating a procedure performed by an eNB forallocating D-ports to a UE in a DAS-based LTE-A system according tostill another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Embodiments of the present invention are described as follows withreference to the accompanying drawings. Detailed descriptions ofwell-known functions and structures incorporated herein may be omittedin order to avoid obscuring the subject matter of the present invention.Further, terms in the following description are defined in considerationof the functionality in the present invention. Therefore, thedefinitions of terms are based upon the overall content of the presentspecification.

Although a detailed description of the present invention is providedwith reference to an OFDM-based mobile communication system, and inparticular, to a 3GPP EUTRA standard, as an example, embodiments of thepresent invention can be applied to other communication systems havingthe similar technical background and channel format through appropriatemodification, without departing from the spirit and scope of the presentinvention.

A cellular mobile communication system is typically implemented bydeploying a plurality of cells in a restricted area. Each cell includesa base station facility placed at the center to provide a mobilecommunication service. The base station facility includes antennas fortransmitting/receiving radio signals and a signal processing part toprovide the mobile communication service to the UEs within the cell.Such a system in which the antennas are concentrated at the center ofthe system is referred to as Centralized Antenna System (CAS).

A Distributed Antenna System (hereinafter, referred to as DAS), bycontrast, is built with antennas distributed within a given cell, i.e.,a service area of an eNB, so as to provide the improved mobilecommunication service as compared to CAS. According to embodiments ofpresent invention, a DAS-based communication system capable ofdistributing antennas within the service area of each eNB and selectingand allocating the antennas efficiently is provided. More specifically,embodiments of the present invention may provide a system control methodthat includes selecting and allocating antennas efficiently in aDAS-based cellular mobile communication system including a plurality ofeNBs, such that each of the eBNs manages the antennas distributed withinthe cell.

As aforementioned, in the CAS-based system, the data rate available forthe UE is significantly influenced by the location within the cell. A UElocated near the center of the cell can be served at higher data ratethan a data rated provided to a UE located at the cell edge. Embodimentspresent invention may address this problem in the cellular mobilecommunication system through a DAS solution.

In the following description according to embodiments of the presentinvention, new terms/concepts are defined, including, but not limited toas D-port, C-port, D-port active set, candidate set, and superset.According to an embodiment of the present invention, the eNB can notifythe UE of the D-port set through UE-specific signaling. After the D-portset notification, the eNB can reconfigure the D-port set according tothe feedback from the UE. The D-port and C-port in the same cell can bedistinguished from each other by according to allocation oftime-frequency resources, and the D-port can be distinguished accordingto separate time-frequency resources or, when multiple D-ports use thesame time frequency resource, the D-ports can be distinguished accordingto scrambling sequences. The received signal strength of a D-port can beused for link adaption and switching between D-ports, and the receivedsignal of a C-port can be used to switch between cells. According to anembodiment of the present invention, if entry of a UE is detected, theeNB informs the UE of multiple available CSI-RS configurations (orsuperset) through UE-specific signaling. Afterward informing the UE, ifmultiple CSI-RS measurement result values (received signal strengthsmeasured by UE) are received from the UE, the eNB transmitsallocation-available transmission positions (i.e., a candidate set)determined based on the CSI-RS measurement result values to the UE.After receiving the candidate set, if the feedback information,including the transmission position information, is received from theUE, the eNB transmits PDSCH data to the UE through the antennas (DASand/or CAS) configured based on the feedback information.

If multiple CSI-RS configurations (i.e., a superset) are receivedthrough UE-specific signaling, the UE measures the received signalstrengths of the configured CSI-RSs and transmits the measurementresults to the eNB. After the measurement results are transmitted, ifthe allocation-available transmission positions set (i.e., a candidateset) is received from the eNB, the UE transmits the feedbackinformation, which includes transmission position information.

According to another embodiment of the present invention, if a UE entryis detected, the eNB, sends, to the UE, a request to transmit an SRS. Ifthe requested SRS is received from the UE, the eNB measures the receivedsignal strength of the SRS transmitted by the UE at distributedpositions. After measuring the received signal strength, the eNBtransmits, to the UE, the transmission position set (or candidate set)configured based on the received signal strength and transmission power.If the feedback information, which includes the transmission positionsinformation, is received from the UE, the eNB transmits PDSCH data tothe UE through the antennas (DAS and/or CAS) configured based on thefeedback information.

Upon receiving the SRS request from the eNB, the UE transmits the SRS tothe eNB. After transmitting the SRS to the eNB, if theallocation-available transmission positions set (or candidate set) isreceived from the eNB, the UE transmits the feedback informationincluding transmission position information to the eNB.

FIG. 3 is a diagram illustrating a configuration of the distributedantenna system-based cellular mobile communication system wheretransmit/receive antennas distributed in each cell according to anembodiment of the present invention.

The following description corresponding to the communication system ofFIG. 3 is directed to an example where each cell includes a centerantenna and four distributed antennas.

Referring to FIG. 3, a cellular mobile communication system according toan embodiment of the present invention includes three cells 300, 310,and 320. Cell 300 is provided with a center antenna 330 positioned at acenter of the cell 300 and four distributed antennas 360, 370, 380, and390 distributed within a service area of the cell 300. The antennas 330,360, 370, 380, and 390 of the cell 300 serve two UEs 340 and 350 toprovide a mobile communication service. The distributed antennas 360,370, 380, and 390 are connected to and controlled by the center antenna330. The connection between the center antenna 330 and the distributedantennas 360, 370, 380, and 390 can be established through variousmethods. The center antenna 330 and the distributed antennas 360, 370,380, and 390, which belong to one cell of the DAS-based system, areconnected to a base station facility so as to be controlled incentralized manner. In the DAS-based system of FIG. 3, the distancebetween the UE 340 and any of the antennas 330, 360, 370, 380, and 390and the distance between the UE 350 and any of the antennas 330, 360,370, 380, and 390 comparatively less than distances betweencorresponding antennas in a CAS-based system. In order to manage theDAS-based system as shown in FIG. 3, the CSI-RS resources for therespective antennas 330, 360, 370, 380, and 390 are assigned.

FIG. 4 is a diagram illustrating a configuration of a resource block forallocating CSI-RS resource to multiple antennas belonging to a cell in aDAS-based system according to an embodiment of the present invention.

The CSI-RS resource allocation of FIG. 4 corresponds to the DAS-basedsystem of FIG. 3, such that the CSI-RS resource elements 400 of FIG. 4are allocated to the center antenna 330 of FIG. 3, while resourceelements 410, 420, 430, and 440 of FIG. 4 are allocated to thedistributed antennas 360, 370, 380, and 390 of FIG. 3, respectively.

In accordance with embodiments of the present invention, building andmanaging of a DAS-based system may be performed in consideration of anew centralized antenna port and a new distributed antenna port, whichare described in detail as follows:

First, a centralized antenna port (hereinafter, referred to as a C-port)is an antenna port that supports channel measurement within an entireservice area of a cell and can be used by both DAS-enabled UEs andnormal UEs. In the LTE-A system, the antenna ports transmitting theCell-specific Reference Signal (CRS) and CSI-RS that can be receivedwithin the entire service area of the cell belong to this category.Second, a distributed antenna port (hereinafter, referred to as aD-port) is an antenna port that can be used by only the DAS-enabled UEand supports the channel measurement in a restricted part of the cell.In the LTE-A system, distributed antennas transmitting the CSI-RSallowing channel estimation in a certain part of the cell belongs tothis category.

In a DAS-based system according to an embodiment of the presentinvention, the physical antennas may be categorized as follows:

First, the centralized antenna (C-Ant) (e.g., antenna 330 of FIG. 3) isplaced at (or near) a center of the cell, and is configured with thetransmit power level and location allowing the transmission signal toreach the edge of the cell. Second, the distributed antennas (D-Ant)(e.g., antennas 360, 370, 380, and 390) are distributed outward from thecenter of the cell, and are configured with transmit power levels andlocations allowing the transmission signal to reach a predetermineddistance within the cell. The C-port and D-port are logical antennasrecognized by the UE independently. However the eNB can use thecentralized antenna or the distributed antennas to implement a specificantenna port or, if necessary, a combination of multiple centralizedantennas or distributed antennas.

According to an embodiment of the present invention, an indexing schemeis defined in order to discriminate among D-ports for supportingefficient DAS communication in the mobile communication standards. TheD-port indexing may be applied to the eNB and UE in the same manner andcan be managed in a cell-specific manner for all of the UEs in a DAS orin a UE-specific manner.

The C-port and D-port can be distinguished from each other by allocatingdifferent time-frequency resources to each port. Since the C-port CRS isallocated different frequency-time resources than those allocated forCSI-RS, no separate resource allocation is necessary for portdiscrimination. Meanwhile, since the CSI-RS can be transmitted by boththe C-port and D-port, a certain time-frequency resource is allocated inorder to discriminate between the C-port and D-port. In the exampleillustrated in FIG. 4, it is possible that the C-port CSI-RS istransmitted in the resource 400 while the D-port CSI-RS is transmittedin the resources 410, 420, 430, and 440.

The C-port and D-port operating in a cell of a system according anembodiment of the present invention may have the following features:

First, the C-port and D-port reference signals area transmitted in therespective time-frequency resources. More specifically, the C-portCSI-RS and the D-port CSI-RS are not be transmitted on the same resourceelement group among the available CSI-RS resource element groups asshown in FIG. 4. Second, the D-port reference signal is transmittedthrough individual time-frequency resources or with different scramblingsequences. More specifically, two D-port CSI-RSs can be transmitted ondifferent time-frequency resources or on the same time-frequencyresource with different scrambling sequences.

The different D-port reference signals can be mapped on the sametime-frequency resource with different scrambling sequences, since theD-port reference signals are transmitted at a transmit power level onlyenough to cover a restricted area. More specifically, the D-portslocated far enough from each other can transmit CSI-RSs on the sametime-frequency resource with different scrambling sequences to randomizethe interference as much as possible. According to an embodiment of thepresent invention, the C-port may be always assigned the time-frequencyresource separately, since the C-port reference signal may betransmitted to cover the entire service area of the cell.

In order to secure the efficient communication in the DAS-based mobilecommunication system, the best C-port and/or the best D-port may beselected.

FIG. 5 is a diagram illustrating the signaling between an eNB and a UEfor allocating D-port in the DAS-based LTE-A system according to anembodiment of the present invention.

Referring to FIG. 5, a UE attempts initial access to a certain cell atthe time point 500. Upon receiving the initial access request, an eNBsends an initial access response signal to the UE in order to notify theUE of successfully performing the initial access at step 505. Once theinitial access succeeds, the UE can connect to a corresponding cell toreceive a downlink signal. After the initial access is performed, theeNB transmits cell-specific CSI-RS related configuration information(e.g., cell-specific CSI-RS and muting information via SIB) to UEswithin the cell, at time point 510. The cell-specific CSI-RS relatedconfiguration information is information broadcast periodically to allUEs within the cell. The information transmitted at time point 510corresponds to the C-port CSI-RS information of the corresponding cell.

The eNB sends, to the UE, a request to transmit an SRS (e.g.,eNB-triggered UE SRS transmission), at time point 515. Upon receivingthe SRS transmission request, the UE transmits the SRS (e.g., a UE SRStransmission), at time point 520. The SRS is an uplink reference signaltransmitted from the UE to the eNB in order for the eNB to check theuplink channel status of the UE. The eNB also can check the averagedownlink channel status by referencing the uplink channel statusmeasured based on the SRS. More specifically, the eNB can send a requestto the UE for the SRS transmission and check the downlink channel statusbased on the SRS transmitted by the UE. The SRS-based downlinkestimation can be implemented in various ways according to embodimentsof the present invention. For example, one approach to performingSRS-downlink estimation is to measure SINR of the received SRS andestimate SNIR of the downlink using the difference between thetransmission powers of the UE and the eNB.

The eNB can determine the downlink based on the SRS transmitted by theUE, at time point 525. The downlinks that may be checked by the eNBinclude the downlink from the D-port of the eNB to the UE as well asfrom C-port of the eNB to the UE. The ability to check the downlinks isenabled by receiving the SRS transmitted by the UE through the antennasconstituting the C-port and D-port of the eNB separately. Morespecifically, if the UE transmits the SRS, the eNB can receive the SRStransmitted by the UE through the C-port and D-ports and analyze thestates of the downlink associated with the C-port and D-ports using thereceived SRS. Using the downlink status obtained in this manner, the eNBselects the D-port for the UE to use at the time point 525.

The information on the D-port selected by the eNB at time point 525 istransmitted to the UE through UE-specific signaling at time point 530.According to an embodiment of the present invention, the UE-specificsignaling carries the information related to D-port assigned to thecorresponding UE, and this information includes the information on theassigned D-port, transmission position of the D-port CSI-RSs, and theinformation necessary for measuring the D-port CSI-RSs and generatingchannel feedback. Upon receiving the information from the eNB, the UEtransitions to the DAS mode to start communication at time point 535.

In FIG. 5, the eNB selects the D-port for a specific UE in the DAS andnotifies the UE of the selected D-port. At this time, the eNB can selectone or more D-ports. In order to manage the DAS-based communicationefficiently, the D-ports are sorted. An example of asorting/classification of the D-ports according to an embodiment of thepresent invention is described as follows:

A D-port superset is a set of all of the D-ports within a cell. Acandidate D-port set is a subset of the D-ports that the eNB informs aspecific UE about, such that the UE in DAS mode can communicate throughsome of the D-ports belonging to the candidate D-port set. An activeD-port set is a set of D-ports through which the eNB communicates withthe UE in a DAS mode and a subset of the candidate D-port set. A requestD-port set is set of the D-ports that the UE in DAS mode requests theeNB to incorporate in the candidate D-port set.

FIG. 6 is a flowchart illustrating a procedure performed by the eNB forallocating D-ports to a UE in the DAS-based LTE-A system according to anembodiment of the present invention.

Referring to FIG. 6, the UE initially attempts to access a cell throughCRS, in step 600. If the initial access request is received from the UEat step 600, the eNB sends an initial access response to the UE. Uponreceiving the initial access response, the eNB sends configurationinformation regarding a CSI-RS for a C-port to the UE, such that the UEreceives the configuration information on the CSI-RS for C-port throughcell-specific signaling, in step 605. The configuration information onthe C-port CSI-RS is cell-specific information broadcast, such that theinformation may be received in an entire service area of a correspondingcell. Once the information on the C-port CSI-RS is received, the UE canperform communication using the C-port of the corresponding cell.

After the UE receives the configuration information, the eNB sends, tothe UE, a request to transmit an SRS, in step 610. Upon receiving theSRS request, the UE transmits the requested SRS such that the eNB canreceive SRS through the central and distribute antennas constituting theD-port, in step 615. The eNB receives the SRS through the central anddistributed antennas, in step 620. The SRS is an uplink reference signaltransmitted from the UE to the eNB, and the eNB can check the uplinkchannel state of the UE based on the SRS and estimate an averagedownlink channel state by referencing the uplink channel state.

The eNB determines whether to configure the UE in DAS mode based on thereceived signal strength of the SRS received per antenna andtransmission power of the C-port and D-port, in step 625. Upon adetermination not to configure the UE in DAS mode in step 625, the eNBand UE communicate only through the C-port, in step 630. However, upon adetermination to configure the terminal in DAS mode, the UE selects acandidate set of D-ports for the UE, in step 635 and transmits, to theUE, CSI-RS configuration for the candidate set of D-ports (UE-specific),in step 640. Once the CSI-RS configuration for the candidate set ofD-ports, the UE can perform D-port/C-port based transmission/reception,in step 645.

The candidate D-port set information provided to the UE includes aplurality D-ports to be used between the UE and the eNB. The UE and/orthe eNB can select one of the D-ports for transmission/reception. Forexample, the eNB notifies the UE of the D-port1, D-port2, and D-port3,and the UE can request use of one of the D-ports referred to in thenotification.

Table 1 corresponds to an example of a D-port subset, candidate D-portset, and active D-port set for use in the system according to anembodiment of the present invention.

TABLE 1 D-port superset Candidate D-port set Active D-port set D-port0D-port0 D-port0 D-port1 D-port1 D-port2 D-port2 D-port3 D-port3 D-port4D-port5 D-port6 D-port7

In Table 1, the D-port superset includes D-port0 to D-port7. The D-portsuperset is a set of all of the D-ports included in the cell. Thecandidate D-port set is a subset of the D-port superset that isdetermined at step 635, and a notification of this determination is sentto the UE at step 640 in FIG. 6. The candidate D-port set is determinedby the eNB and a notification of this determination is sent to the UEthrough RRC signaling. Since the candidate D-port set is informationsent through RRC signaling, changes of the candidate D-port set areperformed in a semi-static manner. The active D-port set is a subset ofthe candidate D-port set that is determined by the eNB in theDAS-related process between the UE and the eNB at step 645 of FIG. 6.The signal delivered from the eNB to the UE is transmitted through thedistributed antennas identical to the active D-port set. The activeD-port set is determined by the eNB and the UE is informed of thedetermined set through separate signaling. When the eNB notifies the UEof the active D-port set, the eNB performs the notification dynamicallyusing Physical Downlink Control Channel (PDCCH) defined in LTE-Astandard. The active D-port set can be managed in a UE-transparentmanner without notifying the UE. More specifically, the eNB transmitsPDSCH data through the active D-port set in communication mode withoutnotifying the UE of the active D-port set such that the UE receives thePDSCH data through the corresponding D-port set. In this case, the UEcan receive the downlink data from the eNB through the distributedantennas in good conditions without a notification sent through thePDSCH data.

Since the configuration information on the CSI-RS for C-port that istransmitted from the eNB to the UE is cell-specific information, theconfiguration information can be broadcast or transmitted in acell-specific manner such that all the UEs can receive the informationwithin the cell. Although according to the examples described herein,the configuration information on the C-port CSI-RS is broadcast ortransmitted in a cell-specific manner, the information also can betransmitted through UE-specific transmission in accordance withembodiments of the present invention.

FIG. 7 is a flowchart illustrating a procedure performed by an eNB forallocating D-ports to a UE in the DAS-based LTE-A system according toanother embodiment of the present invention.

Referring to FIG. 7, steps 700 to 730 are performed in the same manneras steps 600 to 630 of FIG. 6. Therefore, a further description of steps700 to 730 is omitted for clarity and conciseness. Upon a determinationto configure the UE in a DAS mode at step 725, the eNB selects an activeset of D-ports for the UE (where the selected active set is not acandidate D-port set) and notifies the UE of the active set of D-ports,in step 735. In the present example, since the active D-port setincludes only one D-port, the process for selecting one of pluralD-ports between the UE and the eNB is not necessary. After notificationof the active set of D-ports, the eNB transmits CSI-RS configurationinformation for the candidate set of D-ports (which is UE-specificinformation) to the UE, in step 740. Upon receiving the CSI-RSconfiguration information, the UE performs communication using theC-port and D-port (i.e., d-port/C-port based transmission/reception), instep 745.

As described above, upon a determination to configure the UE in a DASmode, the eNB can transmit the information for configuring the candidateD-port set or the active D-port set. Here, the candidate D-port setincludes a plurality of D-ports available for communication between theUE and the eNB such that the UE and/or the eNB can select one of theD-ports for communication. More specifically, if the eNB notifies the UEof the candidate D-port set of D-ports, the UE can request the eNB touse one of the D-ports in the candidate D-port set. When the activeD-port set is transmitted in place of the candidate D-port set, the stepfor selecting on of plural D-ports can be skipped, since, in such acase, the active D-port set includes only one D-port.

FIG. 8 is a flowchart illustrating a procedure performed by an eNB forallocating D-ports to a UE in the DAS-based LTE-A system according tostill another embodiment of the present invention.

Referring to FIG. 8, the UE initially attempts access to a cell throughCRS, in step 800, and the eNB transmits an initial access response tothe UE in response to the initial access request. After the response tothe initial access request is transmitted, the UE receives configurationinformation regarding the CSI-RS for C-port through cell-specificsignaling, in step 805. The configuration information regarding theC-port CSI-RS, which is transmitted from the eNB to the UE, iscell-specific information receivable at any location within the servicearea of the cell. If the configuration information regarding the C-portCSI-RS is received, at step 805, the UE can perform communication usingthe C-port of the corresponding cell.

After the UE receives the configuration information, the UE transmitsthe configuration information on all of the D-ports of the correspondingcell (i.e., a D-port superset) to the UE, in step 810, and the UEreceives the configuration information on the CSI-RS for D-port throughcell-specific or UE-specific signaling. The information received by theUE includes the D-port CSI-RS configuration information and transmissionpower-related information. The UE determines the requested set ofD-ports based on the information transmitted by the eNB, in step 815.The requested D-port set determined by the UE can differ from the finalcandidate D-port set. The UE transmits the requested D-port set to theeNB, in response to the eNB request for a requested set for use in DASmode, in step 820.

If the requested D-port set transmitted by the UE is received, at step820, the eNB determines whether to configure the UE for a DAS mode or aCAS mode, in step 825. Upon determining to configure the UE for the CASmode, the eNB notifies the UE of the CAS mode determination result andperforms C-port only-based transmission and reception, in step 830.Otherwise, upon determining to configure the UE for the DAS mode, atstep 825, the eNB transmits the information on the configuration ofcandidate set and control and setting information necessary foroperation in the DAS mode (i.e., configuration information on CSI-RS forcandidate set of D-ports) through UE-specific signaling, in step 835. Ifthe configuration information is received at step 835, the UE performscommunication using D-port or C-port (i.e., D-port/C-port basedtransmission/reception), in step 840.

In the example according to FIG. 8, the UE can determine a D-portadvantageous to itself by measuring the CSI-RS for the D-port. TheCSI-RS for D-port can be used for determining a channel that hasrelatively superior channel conditions, as well as for measuring thechannel status of the D-port. More specifically, the eNB and UE candetermine which D-port is optimal for DAS using the received signalstrength of the D-port. Meanwhile, a mobile communication systemincluding multiple cells has to support roaming between cells, i.e.handoff. According to an embodiment of the present invention, the mobilecommunication system may always the C-port signal for handoff. Morespecifically, the received signal strength of the D-port is used formovement between D-ports while the received signal strength of theC-port is used for movement between C-ports.

Since the information regarding the C-port is cell-specific information,this information is transmitted such that the information is receivablein the entire service area of the cell. It is also possible to transmitthe information related to C-port in a UE-specific manner according toembodiments of the present invention. For example, in the methodscorresponding to FIGS. 6, 7, and 8, in transmitting the information onthe C-port to the UE, the eNB can transmit the cell-specific informationthrough UE-specific signaling in place of cell-specific signaling.

As described above, an antenna allocation apparatus and method forDAS-based cellular mobile communication system according to embodimentsof the present invention may distribute antennas outward from the centerof each cell, resulting in improving the quality of mobile communicationservice as compared to CAS-based systems. An antenna allocationapparatus and method for DAS-based cellular mobile communication systemaccording to embodiments of the present invention may also be capable ofselecting and allocating the antennas efficiently to improve the systemthroughput. Furthermore, an antennal allocation apparatus and method forDAS-based cellular mobile communication according to embodiments of thepresent invention may guarantee high data rates to the UEs regardless oftheir locations in the cell by configuring and allocating the centraland distributed antennas deployed within the cell efficiently.

While the invention has been shown and described with reference tocertain embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims and their equivalents.

1. A method for allocating antennas performed by a base station in acellular mobile communication system, the method comprising:transmitting, to a User Equipment (UE), information indicating aconfiguration of a plurality of Channel Status Information ReferenceSignals (CSI-RSs) through UE specific signaling during an initial accessattempt with the UE; receiving, from the UE, CSI-RS measurement resultsindicating configured CSI-RSs; transmitting CSI-RSs corresponding to aset of available distributed ports (D-ports) based on received signalstrength information included in results of the CSI-RS measurement; anddetermining a CSI-RS of a selected D-port set for use in communicationsbased on feedback information received from the UE.
 2. The method ofclaim 1, wherein the plurality CSI-RSs corresponding to the transmittedconfiguration correspond to D-ports included in a D-port superset,wherein the available D-port set is a candidate D-port set that is asubset of the D-port superset notified to the UE, and wherein theselected D-port set is an active D-port set including D-ports for use incommunication.
 3. The method of claim 2, wherein the CSI-RSs of theD-ports are transmitted on different time-frequency resources.
 4. Themethod of claim 3, further comprising: transmitting a CSI-RS of aCentral-port (C-port) through cell-specific signaling along with theinformation indicating the configuration for the plurality of CSI-RSs;determining, when the CSI-RS measurement results are received, whetherto configure a Distributed Antenna System (DAS) mode or a CentralizedAntenna System (CAS) mode; configuring, upon determining to configurethe DAS mode, the CSI-RS of the available D-port set; and configuring,upon determining to configure the CAS mode, the C-port.
 5. The method ofclaim 3, wherein the CSI-RS of the D-port set is transmitted through aRadio Resource Control (RRC) signaling.
 6. The method of claim 5,further comprising transmitting Physical Downlink Shared Channel (PDSCH)data to the UE through the active D-port set in a downlink datatransmission.
 7. The method of claim 6, further comprising transmittingthe active D-port set to the UE.
 8. The method of claim 2, wherein theCSI-RSs of the D-ports are transmitted on the same time-frequencyresource with different scrambling sequences.
 9. A method for allocatingantennas performed by a User Equipment (UE) in a cellular communicationsystem, the method comprising: receiving, from a base station throughUE-specific signaling after an initial access attempt with the basestation, information indicating a configuration of a plurality ofChannel Status Information Reference Signals (CSI-RSs); transmitting, tothe base station, received signal strengths of CSI-RSs that are measuredbased on the received information indicating the configuration;receiving, from the base station CSI-RSs of a set of availabledistributed ports (D-ports); feeding back, to the base station, resultsof measurements of CSI-RSs of the D-port set; and receiving, throughdata communication with the base station, downlink data transmittedthrough the D-port set.
 10. A method for allocating antennas performed abase station in a cellular mobile communication system, the methodcomprising: sending, to a User Equipment (UE), a request fortransmission of a Sounding Reference Signal (SRS) during an initialaccess attempt; measuring signal strength of the requested SRS receivedthrough a Central port (C-port) and Distributed ports (D-ports);transmitting, through UE-specific signaling, information indicating aconfiguration of Channel Status Information Reference Signals (CSI-RSs)of available an D-port set configured based on the measured signalstrength and transmission power of the SRS; and determining a CSI-RS ofa selected D-port set based on feedback information received from theUE.
 11. The method of claim 10, wherein the available D-port set is acandidate D-port set that is a subset of a D-port superset including allD-ports of a cell, and wherein the selected D-port set is an active setincluding D-ports for use in communication.
 12. The method of claim 11,wherein measuring the signal strength comprises: determining whether toconfigure a Distributed Antenna System (DAS) mode or a CentralizedAntenna System (CAS) mode based on a result of the measurement;configuring, upon determining to configure the DAS mode, the CSI-RS ofthe available D-port set; and configuring, upon determining to configurethe CAS mode, the C-port.
 13. The method of claim 12, wherein thedetermination of whether to configure a DAS or a CAS is based onreceived signal strengths of the SRSs received through the C-port andD-ports and transmission power information on the C-port and D-ports.14. A method for allocating antennas performed by a User Equipment (UE)in a cellular mobile communication system, the method comprising:transmitting a Sounding Reference Signal (SRS) in response to an SRSrequest received from a base station after an initial access attempt;and transmitting, when Channel Status Information Reference Signals(CSI-RSs) of available distributed ports (D-port set) is receivedthrough UE-specific signaling, feedback information for selectingantennas.
 15. A system comprising: a User Equipment (UE) for receivinginformation indicating a configuration of a plurality of CSI-RSs from abase station through UE-specific signaling after initial access attempt,measuring received signal strengths of the configured CSI-RSs,transmitting measured signal strengths to the base station, receiving anavailable Distributed port (D-port) set from the base station, andmeasuring CSI-RSs of the available D-port set; and a base station fortransmitting the information indicating the configuration of theplurality of CSI-RSs to the UE through UE-specific signaling during theinitial access attempt, analyzing the CSI-RSs received from the UE,configuring the CSI-RSs of the available D-port set based on a result ofthe analysis, transmitting the CSI-RSs of the available D-port to theUE, and determining CSI-RSs of a selected D-port set used forcommunication based on feedback information received from the UE. 16.The system of claim 15, wherein the plurality CSI-RSs corresponding tothe transmitted configuration correspond to D-ports included in a D-portsuperset, wherein the available D-port set is a candidate D-port setthat is a subset of the D-port superset notified to the UE, and whereinthe selected D-port set is an active D-port set including D-ports foruse in communication.
 17. The system of claim 16, wherein the basestation transmits the CSI-RSs of the D-ports on different time-frequencyresources.
 18. The system of claim 17, wherein the base stationtransmits a CSI-RS of a Central-port (C-port) through cell-specificsignaling along with the information indicating the configuration for aplurality of CSI-RSs, determines, when the CSI-RS measurement resultsare received, whether to configure a Distributed Antenna System (DAS)mode or a Centralized Antenna System (CAS) mode, configures, upondetermining to configure the DAS mode, the CSI-RS of the availableD-port set, and configures, upon determining to configure the CAS mode,the C-port.
 19. The system of claim 18, wherein the base stationtransmits the CSI-RSs of the D-port set through a Radio Resource Control(RRC) signaling.
 20. The system of claim 19, wherein the base stationtransmits Physical Downlink Shared Channel (PDSCH) data to the UEthrough the active D-port set in a downlink data transmission.